Key Parameters Influencing ASAG Flooding
6.7 Effect of Tapering
In a typical ASAG injection process, each cycle consists of gas/CO2 and AS slug injection of fixed PV/duration. In tapered ASAG process, size/duration of the chemical and/or gas injection varies in each progressive cycle. It is termed tapering down when the duration of fluid injection is longer initially and decreased in progressive cycles. The reverse is the case with tapering up where the duration of fluid injection is shorter initially and increased in the progressive cycles. Liquid, gas and combined liquid-gas tapering ASAG flooding experiments were performed and compared to examine their effect on oil recovery efficiency. The other operating conditions in these experiments were maintained the same as in the uniform flooding.
0 2 4 6 8 10
0 20 40 60 80
Cumulative oil recovery (% OOIP)
Fluid injected (PV)
2 PV 1.5 PV 1 PV 0.5 PV 2.5 PV Water
flooding ASAG Extended
water flooding
Chapter 6 Key Parameters Influencing ASAG Flooding
Table 6.2: Summary of core flooding experiments evaluating the effects of gas injection rate and total injection volume Sl.
No.
Description of ASAG process
Porosity (%)
K (mD)
Saturation (%)
Fluid Injected (PV)
Recovery (% OOIP)
Kabs Soi Swc WF EOR EWF WF ASAG Total
1
ASAG I
Gas rate (0.2 ml/min)
Injection volume (2 PV) 20.27 6.87 74.21 25.79 4.00 2.00 4.00 35.94 23.85 59.79
2 Gas rate (0.5 ml/min) 20.14 5.85 76.34 23.66 4.00 2.00 4.00 36.86 22.38 59.24 3 Gas rate (1 ml/min) 20.16 6.53 76.53 23.47 4.00 2.00 4.00 35.18 15.86 51.04 4 Gas rate(2 ml/min) 20.21 6.06 75.78 24.22 4.00 2.00 4.00 35.95 15.31 51.26 5 Injection volume (0.5 PV) 20.49 6.10 77.35 22.65 4.00 0.5 4.00 33.95 2.76 36.71 6 Injection volume (1 PV) 20.27 5.99 76.02 23.98 4.00 1.00 4.00 34.28 9.29 43.57 7 Injection volume (1.5 PV) 20.09 6.31 75.23 24.77 4.00 1.50 4.00 33.71 16.61 50.32 8 Injection volume (2.5 PV) 20.52 6.06 75.87 24.13 4.00 2.50 4.00 35.57 24.59 60.16
Chapter 6 Key Parameters Influencing ASAG Flooding
In order to examine the effect of liquid tapering on residual oil recovery, two tapered ASAG flooding experiments were performed and their results were compared with the uniform flooding experiment. The total volume of gas/CO2 and AS formulation alternately injected was fixed at 2 PV. In uniform ASAG flooding as in ASAG I, each cycle of ASAG injection consisted of fixed volume (0.25 PV) of AS slug and CO2 gas injection, whereas, in liquid tapering, the volume of liquid injection was varied in each progressive cycle keeping the gas volume constant. In tapering down, the volume of liquid injection was larger at first and decreased in subsequent cycles. In this injection mode, during the ASAG injection, the gas/CO2 slug size/volume was kept constant at 0.25 PV whereas the AS slug volume was decreased in each progressive cycle in the sequence 0.4, 0.3, 0.2 and 0.1 PV. In the tapering up mode, the liquid volume increased in each progressive cycle in the sequence of 0.1, 0.2, 0.3 and 0.4 PV. Fig. 6.7 shows the cumulative oil recoveries and the ΔP versus the PV of fluid injected during the liquid tapering ASAG flooding experiments. The liquid tapering up ASAG flooding resulted in better incremental oil recovery (27.48 %OOIP) compared to liquid tapering down injection (25.72 %OOIP). However, the additional oil produced by both these tapered injection schemes were higher than the uniform ASAG flooding (23.85 %OOIP). Thus, liquid tapering had a positive effect on the oil recovery by ASAG flooding. In liquid tapering up method, the slug ratio was low in the first cycle which means that the volume of the injected chemical solution was lower while gas/CO2 volume was higher. With higher initial gas saturation, the CO2 injected into the core plug could cause maximum dissolution with crude oil. This resulted in an increase in the relative permeability of residual oil in the core plug probably due to the oil swelling and viscosity reduction caused by the dissolution of injected CO2 gas. When the chemical slug was alternately injected, the displacement efficiency was further enhanced because of IFT reduction and foam formation. As the
Chapter 6 Key Parameters Influencing ASAG Flooding
liquid volume increased towards the end of the ASAG injection during liquid tapering up, the increase in liquid volume helped to control the early gas/CO2 breakthrough by reducing the relative permeability to gas.
To study the effect of gas tapering on ASAG performance, ASAG flooding experiments were performed in gas tapering down and gas tapering up modes. In the gas tapering down ASAG injection, the core plug with residual oil was subjected to 4 cycles of CO2 and AS slug alternate injection. The size of chemical slug injected was kept constant at 0.25 PV in each cycle, while gas slug size was decreased progressively in the sequence 0.4, 0.3, 0.2 and 0.1 PV. In the other experiment, the gas tapering up ASAG injection was applied where the gas slug size was increased progressively in sequence 0.1, 0.2, 0.3 and 0.4 PV. The total PV of fluid injected during the ASAG cycles was kept constant at 2 PV.
The cumulative oil recovery and ΔP as a function of the PV of fluid injected during the gas tapering down and gas tapering up ASAG flooding processes are shown in Fig. 6.8 (a) - (b). The additional oil recoveries of 26.98 % and 24.75 % OOIP were obtained during the gas tapering down and tapering up ASAG injection respectively. These recoveries were better than the recovery by the uniform ASAG injection (ASAG I, 23.85 %OOIP). In comparison, the gas tapering down ASAG injection was found to be superior in terms of oil recovery. By injecting more gas in the first cycle maximum dissolution of CO2 gas with crude oil and efficient use of gas most likely take place leading to the betterment of Evo
[235]. The trapping of the gas/CO2 also resulted in improved oil recovery due to more stable foam formed during the next cycle of AS slug and gas injection [236]. More stable foam formation was evident from the increase in the average ΔP to 68.38 psi (MRF = 1.33) during gas tapering down injection scheme. The average ΔP during the tapering up scheme was 67.25 psi (MRF = 1.30) – higher than the uniform ASAG injection (MRF = 1.27). As the water-to-gas injection volume ratio increases towards the end of the gas tapering down ASAG process, this also helps to control the mobility of the gas [62].
Chapter 6 Key Parameters Influencing ASAG Flooding
The combined effect of applying the gas tapering down and the liquid tapering up injection in a single ASAG flooding process was also examined to observe the enhancement in oil recovery. Following secondary water flooding, the ASAG injection was performed in such a manner that the gas slug size decreased progressively in the sequence of 0.4, 0.3, 0.2 and 0.1 PV, while the liquid volume increased as 0.1, 0.2, 0.3 and 0.4 PV. The cumulative oil recovery and ΔP data are shown in Fig. 6.9 and Table 6.3. The additional oil produced was 29.35 %OOIP, which was an improvement of 5.50 %OOIP over the uniform flooding. Thus, the synergic combination of gas and liquid tapering can be successfully applied to optimize the ASAG flooding through efficient use of the injected fluids. The MRF value also increased in favor of combined tapering ASAG flooding from 1.27 to 1.40 compared to uniform fluid injection. This was an indication of improvement in mobility control which in turn caused better displacement of the residual oil. Srivastava and Mahli [237] showed that the displacement efficiency in both tapering up (20.73%) and tapering down (23.84%) WAG injection process was higher than the normal (19.3%) WAG process. The better recovery was due to improved Evo, better mobility control, and increased oil relative permeability in the pores of the core sample.
Khan et al. [235] in an effort to optimize the miscible WAG process observed the application of tapered injection was more favorable than uniform WAG due to efficient gas utilization, faster recovery rate, and reduction of response time. According to Tovar [238]
the application of tapered WAG could reduce the residual oil saturation from 34.88% to 10.92%. Similarly, Verma [28] reported that tapered WAG is a widely used technique that improves process efficiency, prevents an early gas breakthrough, and improves CO2 utilization.
Chapter 6 Key Parameters Influencing ASAG Flooding
Fig. 6.7: Cumulative oil recovery and pressure drop versus pore volume of fluid injected in ASAG flooding with (a) Liquid tapering up, and (b) Liquid tapering down
0 2 4 6 8 10
0 25 50 75 100 125 150
Pressure drop Cumulative oil recovery Fluid injected (PV)
Pressure drop (psi)
0 20 40 60 80 100
Cumulative oil recovery (% OOIP)
CO2AS CO2AS ASCO2 CO2AS
Water flooding
Extended water flooding (a)
0 2 4 6 8 10
0 25 50 75 100 125 150
Pressure drop Cumulative oil recovery Fluid injected (PV)
Pressure drop (psi)
0 20 40 60 80 100
Cumulative oil recovery (% OOIP)
CO2AS CO2AS ASCO2 CO2AS
Water flooding
Extended water flooding (b)
Chapter 6 Key Parameters Influencing ASAG Flooding
Fig. 6.8: Cumulative oil recovery and pressure drop versus pore volume of fluid injected in ASAG flooding with (a) Gas tapering down, and (b) Gas tapering up
0 2 4 6 8 10
0 25 50 75 100 125 150
Pressure drop Cumulative oil recovery Fluid injected (PV)
Pressure drop (psi)
0 20 40 60 80 100
Cumulative oil recovery (% OOIP)
CO 2AS CO 2AS ASCO 2CO2AS
Water flooding
Extended water flooding (a)
0 2 4 6 8 10
0 25 50 75 100 125 150
Pressure drop Cumulative oil recovery Fluid injected (PV)
Pressure drop (psi)
0 20 40 60 80 100
Cumulative oil recovery (% OOIP)
CO2AS CO2AS ASCO2 CO2 AS
Water flooding
Extended water flooding (b)
Chapter 6 Key Parameters Influencing ASAG Flooding
Fig. 6.9: Cumulative oil recovery and pressure drop versus pore volume of fluid injected in ASAG flooding with combined liquid tapering up and gas tapering down